1. Field of the Invention
The present invention concerns a method and computed tomography (CT) apparatus for operating images of a periodically moving organ of an organism, the organ having regions with rest and movement phases with the rest phases of different regions ensuing at different points in time. The method and CT apparatus are of the type wherein an x-ray source is moved around the body of the organism to be examined to the CT images, to obtain a number of projections serving for the image generation during at least one rotation of the x-ray source around the subject to be examined and during a duration that is at least equal to a period of the motion.
2. Description of the Prior Art
Methods and systems of the type described above are used to generate computed tomography images of the heart that show the heart in a rest phase.
By means of such known methods, it is generally not possible to develop computed tomography images of the coronary arteries that contain little movement artifacts, that are useful for the determination of the degree of calcification or the diagnosis of stenoses in the coronary arteries. The regions of the heart that must be imaged for such examinations in computed tomography images lacking movement artifacts are the right coronary artery (RCA), the left aorta (LM=Left Main), left arterial circumflex (LCX) and the left descending artery (LAD=Left Artery Descendent). For the four cited regions, the speed and the phase of the spatial movement are respectively different during a heart cycle.
Since the position of the diastolic phase of the heart can be evaluated, for example from an ECG signal of the patient acquired during the examination, and the ventricles as well as LM and LAD are largely at rest during diastole, it is a common procedure to acquire ECG-triggered axial scan data for representations of the heart lacking movement artifacts. In addition, it is known to first acquire data and to simultaneously record the ECG signal with the data acquisition, in order to then retrospectively determine, using the ECG signal, the data that were acquired during diastole, and to reconstruct an image based on this data.
The reconstruction of an image of the heart based on data that was determined during diastole, however, in general allows no sharp imaging of RCA and LCX, since their movement in the diastole can be significant. Only with electron beam computed tomography (EBT) (due to the short scan times per slice (looms) with these devices) can a measurement interval sometimes be found in a phase of the heart cycle during which the four cited arteries exhibit only little motion. For patients with higher pulse rates, this does not work for the most part. With conventional computed tomography devices common today, that employ scan times of not less than 330 ms per slice, it is impossible, for patients with lower pulse rates to find a measurement interval in which all cited regions exhibit relatively little motion.